Monday, August 3, 2009 - 4:40 PM

OOS 7-10: Groundwater extraction, fire, and desertification: A case study in Owens Valley, CA

Daniel W. Pritchett and Sara J. Manning. Bristlecone Chapter, California Native Plant Society


As fresh water for human consumption becomes increasingly scarce, groundwater-dependent ecosystems become increasingly threatened, and understanding of their hydro-ecology becomes increasingly important for effective management. The city of Los Angeles pumps an average of about 11,480 ha-m/yr (93,000 ac-f/yr) of water from groundwater-dependent ecosystems in Owens Valley, Inyo County, CA under a joint management agreement known as the Inyo-Los Angeles Long Term Water Agreement (LTWA). We examined trends in grass and shrub cover in one of Los Angeles’ Owens Valley wellfields using monitoring data gathered from 1987 through 2008. We focused on portions of the wellfield in which a groundwater-dependent meadow system was subject to an ad hoc experiment by the imposition of pumping-induced drawdowns of different magnitudes and durations. A wildfire in July 2007 then overlaid an unplanned burn treatment over portions of this area. Ecosystem conditions were documented annually starting in 1987 by repeat photography and cover measurements at both fixed monitoring sites (using a point frame) as well as at locations randomly chosen each year (using point intercept along a 50 m tape). 

Grass-dominated sites in 1987-1988 subject to continuous drawdown below the rooting zone of native phreatophytic grasses (Sporobolus airoides and Distichlis spicata) showed declines in grass cover but increases in shrub cover over time. By the time of the 2007 fire total cover had diminished and the sites had become shrub-dominated. A year after the fire the sites were denuded, with no shrubs, little perennial grass and weedy annuals dominant. Grass-dominated sites subject to minimal groundwater drawdown and subsequent recovery to the grass rooting zone, however, showed an increase in both grass and shrub cover over time. After the 2007 wildfire, robust regrowth of native perennial grass started within a week, and one year later grass cover was comparable to that first measured 20 years before. Fire served as a force for meadow regeneration in the presence of shallow groundwater, but accelerated the ongoing desertification process in areas subject to long term continuous drawdowns. These results are consistent with expectations of the LTWA’s biological management model: that meadow vegetation could tolerate one-to-several-year cycles of drawdown and recovery, but not prolonged drawdowns below the grass rooting zone.